I would comment to drop the preposition: “…only contend two things” I think the trouble in the phrase is that “contend” has an active sense to it whereas “on”

I would comment to drop the preposition: “…only contend two things” I think the trouble in the phrase is that “contend” has an active sense to it whereas “on” creates a more passive tone. Your solution is to swap to a more passive phrasing, but the alternative is also available.